The interest in fuel cell technology as an alternative to internal combustion engines is growing rapidly with the increased concern with environmental issues such as reducing vehicle emissions. The Internal Combustion Engine (ICE) using gasoline produces carbon monoxide and carbon dioxide that are harmful to the environment. The total consumption of petroleum in the U.S. was, in 1997, of the order of 700 billion liters per year. The transportation sector is the single largest user of petroleum, consuming approximately two thirds of the total. About three quarters of this amount is used by automobiles, trucks, and buses, creating a level of pollution in cities that is increasingly unacceptable. The share of highway vehicle polluting emissions is 62% for CO, 32% for NOX, 27% for volatile organic compounds (VOCs), and 20% for CO2. Fuel cells offer a power source that produces electrical energy from fuel and oxidant that produce little or no emissions.
If a fuel cell system is to be operated on hydrocarbon fuels, such as natural gas or methanol, the fuels often have to be converted to hydrogen-rich gas streams by reacting them with steam over a catalyst. This process, called "reforming", requires the expenditure of some energy. The total fuel efficiency of a fuel cell system must then take into account the efficiency of the reforming process as well as the efficiency of the fuel cell. The efficiency of electricity production from hydrocarbon fuels, by fuel cell systems, can range from 40% to 60%. If the heat that is generated as a by-product of the fuel cell is also used, total energy efficiencies for fuel cell power conversion systems can approach 90%. By comparison, modern thermal power stations deliver fuel efficiencies between 30 and 40%.
With cogeneration and adequate heat recovery, thermal power stations can also deliver total energy conversion efficiency reaching 90%. The efficiency of a fuel cell, being proportional to the operating voltage, is greater at lower current and therefore lower power. Thus, unlike heat engines, a fuel cell becomes more efficient at partial load, where most systems are operated. The main types of fuel cell are: